This invention relates to a polishing cloth and more particularly to a polishing cloth suited to the final polishing of hard disk substrates and silicon wafers.
It has been known to use a polishing cloth of the type produced by removing the non-foamed layer (referred to as the skin layer) which constitutes a surface portion of the foamed layer by buffing or by means of a knife so as to expose air bubble cells generated inside the foamed layer on the surface by 1–1000 μm. This was both because it is necessary to hold the polishing liquid in the air bubble cells and because the surface of a foamed resin material normally produced is not sufficiently smooth and flat. Surface roughness due to air bubble cells affects the surface flatness adversely, and it is becoming a serious problem because surface roughness results on surfaces polished by such a polishing cloth. The problem of surface roughness is recently becoming particularly important in the technical field of final polishing of hard disk substrates and silicon wafers and it is becoming essential to reduce such surface roughness.
In view of the problem of reducing surface roughness, Japanese Patent 3,187,769 disclosed the technology of using sandpaper or the like with fine particles to buff the surface of a polishing cloth after air bubbles are exposed, and Japanese Patent Publication Tokkai 2001-62704 disclosed the technology of minutely buffing the skin layer without exposing air bubbles on the surface to improve surface flatness. The former technology is not satisfactory because the surface flatness cannot be improved over a certain limit because air bubbles are exposed on the polishing surface. Neither is the latter technology satisfactory but it is because the polishing liquid cannot be retained.